OPC is known as a conventional technique for optimizing recording power. According to the OPC technique, test data is recorded onto an area of an optical disk known as a Power Calibration Area, or PCA, while changing the level of the recording power. Specifically, a power function (γ) is calculated using a reproduced RF signal relating to data for test recording, and recording power at a level which results in the calculated power function, or a γ value, having a predetermined target value (a target recording power) is then calculated. Further, using the calculated target recording power, the optimum recording power is determined. A γ value is defined as Expression 1, based on a modulation degree m of a reproduced RF signal, and recording power Pw.γ=(dm/dPw)/(m/Pw)  (1)
The right side (dm/dPw) of Expression 1 above is a value obtained by differentiating a modulation degree m by recording power Pw.
In actuality, a target γ, a parameter ρ for use in calculation of the optimum recording power based on a target recording power, an erasing/recording power ratio ε, an erasing/recording power ratio compensation coefficient for low speed recording κ, and so forth are recorded in a read-in area on an optical disk, and read out for use in determination of the optimum recording or erasing power.
That is, using a target recording power Pwt, which can produce a target γ value, the following Expressions (2) to (4) are obtained.Pwo=ρ·Pwt  (2)Peo=ε·Pwo  (3)Peo′=κPwo  (4)
In these expressions, Pwo is the optimum recording power, Peo is the optimum erasing power for twice or four time speed, and Peo′ is the optimum erasing power for a normal speed.
These values, including a target γ value, a parameter ρ, a power ratio ε, and so forth, as determined from an optical disk manufacture under conditions of 25° C., a standard or slower speed, and a laser wavelength 785 nm, are recorded onto a manufactured optical disk.
However, because a modulation degree m for use in calculation of a γ value contains an error due to variation of in-plane sensitivity of a recording film of an optical disk, an error-contained γ value is generally resulted due to the error. This makes it difficult to uniquely determine a target recording power based on a target γ value designated by an optical disk manufacturer, and also difficult to accurately determine the optimum recording power.
FIG. 9 is a graph showing modulation degrees m relative to recording power at respective levels, and variation of a γ value calculated based on a modulation degree using Expression (1). The abscissa of the graph corresponds to a recording power Pw for test recording, while the left ordinate corresponds to a modulation degree m and the right ordinate corresponds to a γ value.
Because of an error contained in a modulation degree m, calculated γ values may fluctuate in the vicinity of a target γ value, which is here set, as an example, at 1.3, as shown in the drawing. That is, a unique level of recording power Pw which produces a target γ value can not readily be determined, if at all.
For example, in the example of FIG. 9, recording power levels P0, P1, P2, and P3 are candidates for the recording power level which can produce a target γ value, or a γ target. As such, the obtained optimum recording power will significantly vary depending on which one of the candidates is chosen for the calculation. In other words, it is difficult to determine the inherent optimum level in this method, which makes it difficult to maintain preferable recording quality (a wave jitter, an error rate, and so forth). Specifically, recording power at a level significantly lower than the inherent optimum level may adversely affect jitter and error rate, while recording power at a level significantly higher than the inherent optimum level may adversely affect reliability for repetitive recording.